Question

I’m working on a program that searches entire drives for a given file. At the moment, I calculate an MD5 hash for the known file and then scan all files recursively, looking for a match.

The only problem is that MD5 is painfully slow on large files. Is there a faster alternative that I can use while retaining a very small probablity of false positives?

All code is in C#.

Thank you.

Update

I’ve read that even MD5 can be pretty quick and that disk I/O should be the limiting factor. That leads me to believe that my code might not be optimal. Are there any problems with this approach?

        MD5 md5 = MD5.Create();         StringBuilder sb = new StringBuilder();         try         {             using (FileStream fs = File.Open(fileName, FileMode.Open, FileAccess.Read))             {                 foreach (byte b in md5.ComputeHash(fs))                     sb.Append(b.ToString('X2'));             }             return sb.ToString();         }         catch (Exception)         {             return '';         } 

Answer 1

I hope you’re checking for an MD5 match only if the file size already matches.

Another optimization is to do a quick checksum of the first 1K (or some other arbitrary, but reasonably small number) and make sure those match before working the whole file.

Of course, all this assumes that you’re just looking for a match/nomatch decision for a particular file.